Henri Pirenne's reputation today rests on two contributions to European history:

what has become known as the Pirenne Thesis, concerning when the Middle Ages started

a distinctive view of Belgium's medieval history

Henri Pirenne first developed the idea for the Pirenne Thesis in a series of papers written from 1922 to 1923, and then spent the rest of his life refining the thesis with supporting evidence. The most famous exposition appears in his book Mohammed and Charlemagne (The Birth of the Occident, The Fall of the Antiquity, and the Rise of the Germanic Middle Ages) first published in 1937 (1939 in English).

Traditionally, historians have dated the Middle Ages from the fall of the Roman Empire in the 5th century, a theory Edward Gibbon famously put forward in the 18th century. Pirenne challenged the notion that Germanic barbarians had caused the Roman Empire to end, and he challenged the notion that the end of the Roman Empire should equate with the end of the office of Emperor in Europe, which occurred in 476. He pointed out the essential continuity of the economy of the Roman Mediterranean even after the barbarian invasions, that the Roman way of doing things did not fundamentally change in the time immediatly after the "fall" of Rome. Barbarians came to Rome not to destroy it, but to take part in its benefits, they tried to preserve the Roman way of life.

According to Pirenne the real break in Roman history occurred in the 7th century as a result of Arab expansion. Islamic conquest of the area of today's Turkey, Syria, Palestine, Spain and North Africa ruptured economic ties to Europe, cutting the continent off from trade and turning it into a stagnent backwater, with wealth flowing out in the form of raw resources and nothing coming back. This began a steady decline and impoverishment so that by the time of Charlemagne Europe had become entirely agrarian at a subsistence level, with no long-distance trade. Pirenne says "Without Islam, the Frankish Empire would have probably never existed, and Charlemagne, without Muhammad, would be inconceivable".

Pirenne used quantative methods in relation to currency in support of his thesis. Much of his argument builds upon the disappearance of items from Europe, items that had to come from outside Europe. For example, the minting of gold coins north of the Alps stopped after the 7th century, indicating a loss of access to wealthier parts of the world. Papyrus, made only in Egypt, no longer appeared north of the Alps after the 7th century: writing reverted to using animal skins, indicating an isolation from wealthier areas.

Pirenne's Thesis is not without critics. One does not have to entirely accept or deny his theory without finding useful elements for understanding the period; and it provides a new way at looking at periodization schemes that have traditionaly been seen as accepted facts.

Pirenne's other major idea concerned the nature of medieval Belgium. Belgium as an independent nation state had appeared only a generation before Pirenne's birth; throughout Western history, its fortunes had been tied up with the Low Countries, which now include the Netherlands, Luxembourg and parts of north-east France. Furthermore, Belgium lies athwart the great linguistic divide between French and Dutch. The unity of the country might appear accidental, something which Pirenne sought to disprove in his History of Belgium (1899 - 1932). His ideas here have also proved controversial, with many historians preferring to stress the economic unity of the Low Countries as a whole.

References

Hodges, Richard and David Whitehouse. Mohammed, Charlemagne, and the origins of Europe. Cornell University Press. 1983. ISBN 0801492629 - influential analysis of the Pierenne Thesis and the role of recent archaeological findings.

McCormick, Michael. Origins of the European Economy: Communications and Commerce, 300-900. Cambridge University Press. 2001. ISBN 0521661021 - a reexamination of the Pierenne Thesis.